1. Field of the Invention
The present invention relates to a step tablet for determining the optimum degree of exposure of photosensitive materials and, in particular it relates to an improved step tablet useful for determining the optimum degree of exposure of peel-apart (delamination) type photosensitive materials more accurately than conventional step tablets.
2. Description of the Prior Art
In a conventional image forming process using a material called a "photosensitive resin" or a "photopolymerizable resin", a wet process is employed utilizing the property of the photosensitive resin in which the photosensitive resin which is originally soluble in a solvent becomes insoluble in the solvent upon light exposure, that is, a solution of the photosensitive resin is coated on a support for image formation, such as a metal plate, etc., followed by drying and after exposing the coated layer through an original to actinic radiation, the coated layer is dissolved off at the unexposed areas only using a solvent which dissolves the coated layer at the unexposed areas but does not dissolve the coated layer at the exposed areas, or, in another embodiment, a photosensitive sheet composed of the photosensitive resin is laminated on the surface of a support for image formation and then the laminated layer is exposed and developed as in the above-described process.
The image forming process as described above involves difficulties in that a large amount of solvent is required for the development and also a long period of time is required for finishing the development. Therefore, to overcome the technical, economic, and hygienic disadvantages in the above-described process, a dry developing process wherein a peel-apart development type photosensitive material comprising a support having laminated thereto a photosensitive layer and a peelable film (i.e., a flexible film which can be separated by peeling off) is exposed through an original to actinic radiation and then the peelable film is separated from the laminated assembly by peeling apart to form an image without using any solvent.
Examples of peel-apart development type photosensitive materials are described in, for example, Japanese Patent Publication Nos. 9663/'63 and 15,932/'66; and U.S. Pat. Nos. 3,353,955 and 3,770,438. For example, in the technique disclosed in Japanese Patent Publication No. 9663/'63, a photosensitive material comprising a support such as a metal sheet, a paper, a synthetic resin film, etc., as the lowermost layer, a film (corresponding to a peelable film) as the uppermost layer, and, as an interlayer between the support and the peelable film, a photosensitive layer, composed of a material in which the bonding strength to the peelable film is higher than that to the support but the bonding strength to the support, when the material is exposed to actinic radiation, becomes higher than that to the peelable film, is exposed through an original to actinic radiation and then the peelable film is separated by peeling apart to leave wholly or partially the unexposed portions of the photosensitive layer adhered to the peelable film and the exposed portions of the photosensitive layer adhered to the support. The above-described technical principle may also be utilized in a reverse manner.
The dry development process using such a peel-apart development system is markedly superior to the above-described wet development process from the standpoint that the development procedure is simple and further since an organic solvent, an aqueous solution, etc., are not used in the dry process, the possibility that dangerous accidents will occur and the necessity for processing waste solutions to avoid environmental pollution are both eliminated. In practice, the dry development process has been employed for making printed electric circuits using a material composed of a metallic layer formed on an electrically insulating material or for making lithographic printing plates using surface-treated aluminum sheets, etc., as the support plate.
To determine the degree of exposure or the optimum exposure time (i.e., the optimum exposure amount to be applied determined from the results of exposure and development) of an image-forming material for the wet development process, a step tablet, for example, Kodak Step Tablet No. 3 (made by Eastman Kodak Co.) is contact-printed on a presensitized plate and in this case, the optimum degree of exposure or amount of exposure for the photosensitive material is determined by the amount such that the part of the photosensitive layer of the pre-sensitized plate corresponding to the 0.9 density step, i.e., the sixth step of the step tablet from the lower density side, forms a solid black image.
However, when such a conventional method using a step tablet is applied to a peel-apart development type photosensitive material for determining the optimum degree of exposure, the following difficulties are encountered.
(1) The position of the density step forming an all-over image differs depending on the direction in which the peelable film of the photosensitive material is peeled apart at development by peeling.
(2) When the peelable film of the photosensitive material is peeled apart from the lower density portion of the step tablet toward the higher density portion, two or more density steps appear as a solid black image, which results in it being impossible to determine accurately the optimum degree of exposure.
To overcome the above-described difficulty (1), establishing a definite peeling direction for the photosensitive material could be considered but such an attempt involves considerable inconvenience in the working procedures. Also, since the degree of exposure must be determined by the number of the density steps forming the all-over image which appear inaccurately as in the above-described difficulty (2), it is difficult in this case, also, to determine accurately the optimum degree of exposure for the photosensitive material.